README doesn't make capabilities clear

[mdales]

In addition to #56 - I also gave up quickly on using this library because it isn't clear from the README that it provides data type abstractions for all the Geometry types etc. That is hidden in geojson_intf.ml, and is implied by some of the examples, but never clearly shown.

As a new user looking at this versus just JSON Decoding by hand, it'd be good to have up front a list of types or an example of using it to extract some features from a data set more clearly. Most of the README is about advanced usage rather than the nuts and bolts of getting started.